1910] ROBBINS—VEGETATION IN COLORADO 269 
limit of distribution in gulches rather than on ridges, and low altitude 
forms extend farther up the ridges than up the gulches. ; 
At certain periods, inversion of temperature of more than local 
influence occurs along the eastern edge of the foothills, which up to a 
considerable altitude experience a higher temperature than the plains. 
Loup? points out that on the eastern slope of the Rocky Mountains in 
Colorado he has often noticed that elevated stations retain a mild 
temperature for some time after a cold wave has set in at a lower level. 
There is much need of thorough investigation of the amount of 
temperature inversion for various localities; of determining its 
limits, time of occurrence, and effect upon the vegetation. There is 
little doubt that an investigation of such a nature would reveal informa- 
tion of practical importance to horticulturists and throw light on 
problems of plant distribution. 
Frost.—The average date of the last killing frost in spring, of the 
first killing frost in autumn, and the average length of the growing 
Season (period without frost) are important climatic factors. In 
Colorado these factors are affected to a considerable degree by 
temperature inversion, and have a great range on account of altitudinal 
differences. Grand Junction has the longest growing season of any 
locality in the state (6 months and 18 days), while on the highest 
peaks there is no day in the year without frost. 
TABLE VI 
EFFECT OF ALTITUDE UPON AVERAGE LENGTH OF GROWING SEASON 
AND AVERAGE DATE OF LAST KILLING FROST 
Average length of grow- 
: A date of last . = 
Altitude (feet) ~ eee tind ing oon a 
5000 and below ...... April 28 5 months, 6 days 
§900-0000. 45s) May 5 4 months, 21 days 
Gope-7606 6 oo. May 30 months, 21 days 
7000-8000 P55 eo F, June 6 3 months, 6 days 
There are no data for higher altitudes. During the summer of 
1908, the length of the growing season at Redrock Lake, altitude 
T0,100 feet, was about 6 weeks. Table VI shows that the retardation 
es the development of vegetation is about 13 days for every 1000 feet 
crease in altitude. 
° Loup, F. H., The Colorado sky I:1-9. 1908. 
